Traditional boat hull design has evolved over many years with each new design being a development of existing accepted concepts. Traditionally, fast displacement and planing boat hull designs are based on an immersed hull form travelling through the water where the ‘V’ hull form for high speed vessels is accepted as being the optimum hull form for a variety of sea states. Designers constantly face challenges related to the ‘V’ hull form where each design attribute presents inherent compromises between that attribute and one or more others.
For example, a hull having a narrow entry will provide reduced form resistance and subsequently be able to travel faster through the water. The compromise being that the narrow bow generates less lift, has less volume, and subsequently less buoyancy, and therefore carries less payload and can have a tendency to submarine through swells. To counter this, designers incorporate a flared bow section. The introduction of the flared bow does not increase payload but introduces additional internal volume high up in the bow, and subsequently provides exponential reserve buoyancy in the bow to provide a hull form that is intended to ride over a swell. The flared bow also increases deck area and helps prevent excessive amounts of water washing across the deck, producing a ‘drier’ boat. The compromise with this form is that when the bow buries into a large swell, the hull presents a relatively blunt form to the swell which increases wave-making resistance and partially stalls the vessel's forward motion.
With regard to planing vessels, it is accepted that the easiest way to get a vessel to plane is to provide a hull with a flat underbody. The compromise with a flat underbody is that the vessel is only suited to flat water operation due to poor performance and excessive slamming loads in rough water. In addition, a planing hull requires design attributes to promote lift which is essential for the hull to transition from displacement mode to planing mode and to continue to operate in planing mode.
The reduction of slamming loads and generation of lift is generally achieved with conventional hulls by the incorporation of ‘V transverse sections below the waterline. With constant deadrise hulls, the ‘V is constant along the length of the vessel which often results in a compromise between accommodating slamming loads at the bow and providing optimum planing performance. Variable deadrise hulls generally transition from a deep, fine ‘V forward, to a flatter ‘V aft. The fine ‘V forward section reduces slamming loads but also reduces the lift required for the vessel to transition from displacement mode to planing mode. The flatter ‘V sections aft provide the planing surface. For operation, essentially, the application of power ‘squeezes’ the bow out of the water, inclining the hull to allow it to be driven out of the water and onto the plane. This squeezing motion propagates equal and opposite forces on both sides of the hull as evidenced by the bow wave, and is representative of the wave-making resistance of the bow portion of the hull. These wavemaking forces constitute large energy losses.
The aft ‘V planing surfaces of a planing vessel provide the planing area required for the hull to ‘ride’ on the water when operating in planing mode. These planing surfaces must maintain an incline in the direction of travel to produce the lift required to stay ‘on the plane’. The combination of the incline and the ‘V hull form propagates an outflow of water from under the hull, as the vessel travels forward, displacing large volumes of water from under the hull. Consequently, the inclined bow buries the stern, creating a void in the water immediately aft of the hull as a result of water being displaced by the vessels motion. This void is evidence of large energy losses.
The incline also increases the propulsion thrust line angle from the horizontal, which reduces the efficiency of the vessel as a whole. Inclined planing vessels also present a relatively broad slamming area to the face of oncoming swells further reducing efficiency, reducing comfort and introducing unnecessary slamming loads and stresses to the vessel.
Planing hull designs, having broad planing aft sections, vary greatly in design from displacement hulls, having diminishing displacement aft, and operate most effectively at the speeds for which they were designed. Displacement hulls by design cannot operate as a planing vessel. However, a planing hull will operate in displacement mode at low speed which is a compromise of the design intent as it operates with a buried stern resulting in increased eddy-making resistance and reduced efficiency.
In addition to the water being displaced by the hull form, water is also displaced by the propulsion system through propellers, water jets or similar. Displacing or relocating water consumes energy, therefore the lower the volume of displaced water and the shorter the displacement distance, the lower the energy required.
In addition to the above, frictional resistance, which is approximately proportionate to the wetted surface area, is inherent in all hulls immersed in water while travelling through it. A reduction in wetted surface area is one of the main focuses of hull designers in an effort to reduce frictional resistance which is one of the greatest contributors to energy losses in boat hulls. To this end, attempts have been made to introduce air bubbles and air films between the hull and the surface of the water with limited practical success.
It is also highly desirable to have a shallow draft vessel to access remote areas especially when running for cover in heavy weather where shallow water may be the only safe refuge. The nature of a ‘V hull is that they are immersed deeper into the water which exposes the underside of the hull and running gear to potential collisions with the bottom.
Preferred embodiments of the present invention seek to overcome or ameliorate one or more of the above mentioned challenges, or at least provide a useful alternative.